This invention relates to a new process for attenuating or removing a silicon coating from the inside surface of a tube. The process of this invention is particularly useful in the art of producing polysilicon granules, which granules are convertible to monocrystalline silicon.
High purity monocrystalline silicon is in great demand as a semiconductor material. The purity of the silicon is critical as impurities, especially metal and hydrogen, can render the silicon unacceptable for some electronic uses.
Most of the world's supply of semiconductor grade silicon is produced from polycrystalline silicon, i.e., polysilicon, which in turn is produced from the thermal decomposition of a silicon source, e.g., silane, trichlorosilane and the like. The thermal decomposition can conveniently be carried out in a fluidized bed reactor into which is fed the silicon source material and a carrier or fluidizing gas. Such processes are exemplified in U.S. Pat. Nos. 4,784,052, 4,784,840, 4,868,013 and 4,883,687, which patents are all incorporated herein as if fully set forth. The polysilicon product from these processes is a free flowing powder comprised of essentially spherical polysilicon granules having a particle size range of from about 150 to about 3000 microns. Average particle size is from about 600 to about 1100 microns.
The fluidized bed process provides for a periodic discharging of the finished polysilicon granules from the reactor after the granules have reached a desired average size. This discharging occurs via a product withdrawal tube which is mounted in the bottom of the reactor. The mounting is conveniently achieved by fixing the upper end of the tube to the distributor plate, through which plate the fluidizing gas is introduced into the reactor. The product withdrawal tube will usually have a narrow inside diameter, say from about 1.5 to about 2.5 inches.
While the fluidized bed technology is advantageous in many respects, it is not without its drawbacks. When silane, a preferred silicon source material, is thermally decomposed in the reactor, the resultant silicon can deposit heterogeneously, i.e., deposit on a surface, or it can deposit homogeneously, i.e., form a silicon dust without a substrate. The former is the mechanism by which the desired granular product is grown. The initial growth, i.e., depositing, occurs on the silicon seed particles fed to the reactor. Unfortunately, this deposition occurs not only on the seed particles but also on the reactor and product withdrawal tube surfaces as well. Facilitating this deposition is the fact that these surfaces are generally of a material which is silicon related. The use of silicon related materials is preferred since such use promotes the obtainment of a very pure final polysilicon product.
The silicon deposition on the reactor and tube surfaces can continue over time until the coating builds to a thickness which adversely affects the functional characteristics of the reactor and/or tube. When the coating becomes too thick, the practice has been to shut the reactor down and to remove or reduce the coating by etching the coating with a mineral acid, such as hydrochloric acid. This etching occurs slowly, but can be accelerated by heating the silicon coating to a temperature of about 500.degree. to about 1000.degree. C. The coating on the reactor walls can be heated by simply using the same equipment used to heat the reactor for the thermal decomposition of the silane to silicon. Heating of the coating on the inside surface of the product withdrawal is more problematical. When the equipment used to heat the reactor is relied upon to heat the interior of the tube, the heat transfer access between the reactor and tube is poor at best. Convective heat transfer is not effective since the mineral acid, which is introduced at the top of the reactor, does not have sufficient heat capacity to heat the product withdrawal tube to the necessary temperature. Conductive heat transfer, while it does occur to some extent, is not favored as the distributor to which the tube is mounted is cooled to prevent its damage. Radiant heat transfer is frustrated by the geometry existing between the reactor and associated product withdrawal tube. With the product withdrawal tube at a relatively low temperature, the etching occurring therein is slow and can extend the time needed to effect the coat reduction sought.
One technique used to speed up the etching in the tube is to dismantle the tube and remove it from the distributor so that the tube or at least portions thereof can be conventionally heated away from the reactor. While this technique can work some of the time, it is not a panacea. Many times the tube is not easily removed from the distributor due to the extent of the silicon build-up. (Generally the silicon build-up will be worst at the distributor-tube juncture.) Also, the product withdrawal tube will almost always be of a fragile material, such as silicon carbide, silicon, quartz, etc., which can make tube damage a real concern.
Therefore, there is a need for a process for the attenuation or removal of a silicon coating from the inside wall of a product withdrawal tube, which process does not require removal of the tube from its association with the fluidized bed reactor and which does not put the tube in jeopardy of being damaged.